Power-assisted steering system

ABSTRACT

A power-assisted steering system for a vehicle comprising a hydraulic motor (1) for providing vehicle steering assistance, an open center servo-valve (6) for supplying the hydraulic motor (1) with hydraulic fluid, a hydraulic pump (7) for supplying hydraulic fluid to the servo-valve (6) and to the hydraulic motor (1), and an electric motor (13) for driving the hydraulic pump (7). The hydraulic pump (7) is controlled by controlling electrical power consumed by the electric motor (13). At least one sensor (15) repetitively monitors at least one operating parameter of the power-assisted steering system. At least one microprocessor (17) repetitively calculates reference values that vary as a function of a monitored level of the at least one operating parameter. Control of the electrical power consumed by the electric motor (13) is based on a comparison of reference values calculated as a function of a first monitored level of the at least one operating parameter with a second subsequent monitored level of the at least one operating parameter. The at least one microprocessor (17) is operable to recalculate the reference values as a function of the second monitored level of the at least one operating parameter based on a comparison of the first monitored level of the at least one operating parameter with the second monitored level.

BACKGROUND OF THE INVENTION

The invention pertains to a power-assisted steering system with aservo-valve that is open at its center and serves for supplying anactuator element with hydraulic pressure, and with a pump that is drivenby an electric motor and serves for supplying the servo-valve withhydraulic fluid, with the power consumption of the electric motor beingcontrolled by an electronic unit as a result of the comparison betweenoperating parameters that are monitored by sensors and fixed referencevalues.

One known type of servo-valve that is utilized in power-assistedsteering systems is the servo-valve with a so-called open center. In theneutral position of a servo-valve with an open center, hydraulic fluidis pumped at low pressure through the open valve into a reservoir bymeans of a pump. When turning the input shaft of the servo-valverelative to the valve sleeve, hydraulic pressure is supplied to ahydraulic motor such that the movement of the steering rack of thesteering system is assisted in one of the two possible directions. Inthese known servo-valves with an open center, the pump for supplyinghydraulic fluid to the servo-valve is driven by the motor of the vehiclevia a V-belt. One disadvantage of these known servo-valves with an opencenter is that a continuous, strong flow of the hydraulic medium ismaintained even in the neutral position of the servo-valve because thepump is directly connected to the motor of the motor vehicle.Consequently, some of the power generated by the motor is lost due tothe dynamic pressure and the volume flow. One additional disadvantage ofknown power-assisted steering systems is that the power-assistance ofthe steering system is deactivated as soon as the motor of the motorvehicle is turned off, e.g., when towing a stranded vehicle.

DE-OS 3,622,217 discloses a power-assisted steering system in which anelectric motor is used for driving the pump. In this knownpower-assisted steering system, the hydraulic pressure is monitored bymeans of a pressure sensor, with the pump drive motor being switchedinto at least one of three positions, namely on, minimum and maximum, independence on the hydraulic pressure.

One additional power-assisted steering system of this type is known fromDE-OS 3,920,862. This system also utilizes an electric motor for drivingthe pump, with said electric motor being additionally controlled by asignal that depends on the speed of the motor vehicle.

These known power-assisted steering systems are based on the idea thatthe electric motor only needs to be supplied with a minimum of energy aslong as the servo-valve is situated in the neutral position. In thiscondition, the electric motor only need drive the pump such that aminimum hydraulic pressure is maintained. When turning the steeringwheel, a higher hydraulic pressure is required, i.e., the electric motoris switched into an operating state of higher power consumption. In thiscase, it is, in principle, desired that this switching take place independence on an input torque that is applied to the input shaft of theservo-valve. In order to realize this with respect to the switchingtechnology, it is, for example, possible to measure the powerconsumption of the electric motor, which represents a measure of therequired hydraulic pressure. Below a certain nominal value of thehydraulic pressure, the electric motor operates in the lower powerconsumption range, with the switching into the higher power consumptionrange taking place once the nominal pressure value has been reached.

However, all these known power-assisted steering systems have thedisadvantage that a torque-dependent switching point cannot be insured.The progression of the required pressure as a function of the inputtorque has the shape of a parabola. It is known that this parabola whichnormally-intersects the pressure axis initially shows only a flatincrease below a certain limiting pressure. Slight differences betweensuch systems, e.g., due to manufacturing tolerances or wear, can alreadyinfluence the position of the parabola on the pressure axis. Inparticular, the temperature of the hydraulic oil significantlyinfluences the position of the pressure curve as a function of thetorque. If the oil is very cold and has a very high viscosity, thepressure in the neutral position of the servo-steering valve is alreadyvery high. If the switching point between the power consumption rangesof the electric motor is defined as an absolute pressure value independence on this pressure minimum, said switching point inevitablytravels on the pressure curve as the hydraulic oil heats up because thepressure required in the neutral position drops. This leads to the factthat, after a certain operating time, the switching of the electricmotor from one power consumption range to another takes place at a latertime, i.e., beginning at a higher torque. Consequently, atorque-dependent switching point cannot be insured.

One additional disadvantage of known power-assisted steering systems canbe seen from the fact that the switching of the electric motor betweendifferent power consumption ranges only takes place in dependence on therequired hydraulic pressure. This leads to the fact that the motor isswitched into the higher power consumption range beginning at a certaintorque-dependent and remains in this higher power consumption rangeuntil the torque-dependent is again reduced. If turning the steeringwheel in a motor vehicle in the vicinity of a curb, it may happen, forexample, that the tires contact the curb or the steering wheel is turnedto its limit despite the fact that a maximum torque is applied and themotor operates in the highest power consumption range. If this conditioncontinues for a sufficient time, the temperature of the motor isunusually increased and can cause destruction of the control electronicor the motor.

Known power-assisted steering systems of this type also are associatedwith the disadvantage that it is very difficult to monitor theirfunctions. Malfunctions usually can only be detected once one of thestructural elements of the power-assisted steering system fails. Thisusually leads to a complete pressure loss, and consequently a loss ofthe power-assist function of the steering system.

The previously described disadvantages cause severe load on the pumpdrive motor and the corresponding sensors as well as the electronics ofthe power-assisted steering system such that high wear and inferiorservice life result. In addition, the displacement of thetorque-dependent switching point also has a disadvantageous influence onthe operating comfort.

SUMMARY OF THE INVENTION

The invention is based on the objective of improving power-assistedsteering of this type in such a way that lesser wear and consequently aprolonged service life is attained with less load on the structuralelements of the power-assisted steering system and improved operatingcomfort. In addition, the invention discloses a method for controllingthe electric pump drive motor.

According to the invention, this objective is attained connectingsensors to a control unit that comprises at least one microprocessor anddefines reference values in dependence on monitored operatingparameters.

This solution according to the invention makes it possible to definereference values that represent the switching limits in dependence onoperating parameters, i.e., based on the monitoring of operatingparameters. It is, for example, possible to carry out a temperaturecompensation of the switching point between a minimum and a maximumpower consumption range. Manufacturing tolerances, wear and the like canbe compensated correspondingly because it can be insured that theswitching point actually is torque-dependent in the desired fashion.

The invention advantageously proposes that the electronics comprise acurrent sensor that measures the power requirement of the pump. Thepower consumption of the electric motor represents a measure of thepower requirement of the pump. During an excursion of the servo-valve, ahigher hydraulic pressure needs to be generated by the pump which, inturn, causes a higher power requirement of the pump. The sensor measuresthis power requirement of the pump and the electronic control unitallows a switching transistor to connect the motor to the full operatingvoltage as soon as the required power exceeds a limiting value. In theother instance, the electronics switch the supply voltage of theelectric motor back into the standby mode once the power requirement ofthe pump drops below a limiting value. In this case, the control forswitching off the supply voltage is designed in such a way that thepower requirement of the pump needs to fall short of the limiting valuefor a certain duration in order cause the supply voltage to switch off.This minimum duration is provided so as to not impair the steeringcomfort due to the fact that the pump is started and stopped againwithin short intervals such that the hydraulic pressure in theservo-valve varies intensely. A current sensor makes it possible for apower-assisted steering system according to the invention to react verysensitively to the power requirements of the pump. According to oneadvantageous embodiment of the invention,-the control unit determines aminimum power consumption of the electric motor by means of the currentsensor so as to define a reference value for the switching point betweencertain power consumption ranges. This measure insures that the powerrequirement in the neutral position of the respective operating stateserves as the basis for calculating the switching point. Consequently,temperature variations and the like cannot have disadvantageous effectson the switching points, i.e., the switching into each operating statecan take place based on the same occurring torque.

According to additional proposals of the invention, the electronics cancomprise a pressure sensor that measures the pressure variations of theservo-valve as soon as an excursion of the servo-valve from the neutralposition takes place or a proximity sensor scans the position of onestructural component in the steering gear and measures the operatingstate of the servo-valve with the aid of the position of said structuralcomponent. The pressure sensor makes it possible to directly definehydraulic pressure parameters. A proximity sensor would, for example,make it possible to determine the excursion of a reaction piston in theservo-valve and consequently a torque-dependent initial value. In alltypes of sensors, the sensor signal is evaluated in an electroniccontrol unit that determines the supply voltage of the electric motortherefrom. A comparison between the current sensor signal and thecorresponding predetermined limiting values is carried out in thiscontrol unit. The operating state of the electric motor is switched tofull power or standby as soon as the sensor signal exceeds or fallsbelow a limiting value.

According to one proposal of the invention, the electric motor operatesin standby mode when the hydraulic cylinder is in the neutral position.Due to this minimum voltage or the standby mode of the pump, thepower-assisted steering system according to the invention has a veryshort reaction time; consequently, power-assistance is available withalmost no delay during an excursion and in the neutral position. If theelectric motor and consequently the pump would be switched off in theneutral position of the servo-valve, the motor speed would have to beaccelerated from zero to full speed during an excursion of theservo-valve such that no power-assistance would be available at thebeginning of the steering movement. According to one additionaladvantageous proposal of the invention, the control unit activatescurrent limiting of the electric motor once a selected operatingparameter has exceeded a predetermined limiting value for apredetermined duration. The value to which the current is limited canlie significantly below the limiting value. This measure makes itpossible to prevent the aforementioned load on the structural componentsas well as the possible destruction of said components while thesteering wheel is turned to either limit stop. According to oneadditional proposal of the invention, the electronic comprises a Hallsensor. This Hall sensor can be used for determining the rotationalspeed of the shaft of the electric motor or rotational movements of theshaft in general. It is, for example, possible to determine if a changein the torque-dependent pressure requirements takes place afteractivating the current limiting such that the system can be switchedback into the normal operating state.

According to one additional proposal of the invention, the control unitmonitors at least two operating parameters as part of a plausibilitycheck. The control unit defines at least two operating parameters thatusually are functionally related, e.g., the rotational speed of theshaft of the electric motor and the hydraulic pressure, via the sensors.Both operating parameters mast be related to one another. This can, forexample, be realized due to the fact that the control unit carries out acomparison to value tables. If this relation does not exist, one canconclude that functional problems are present. This measure also makesit possible to detect functional problems early. The control unit isadvantageously provided with a memory for storing the value tables,intermediate values, program routines and the like.

The invention proposes a method for controlling the electric pump drivemotor of a power-assisted steering system of this type in whichreference values are defined in dependence on at least one monitoredoperating parameter so as to carry out a comparison to monitoredoperating parameters.

The method according to the invention insures that reference values,e.g., switching values, can be adapted to the respective operatingstates.

It was explained previously that servo-steering valves of this type aremanufactured with certain variations in the characteristic pressuredifference/torque line. The power consumption of the electric motor ismainly controlled within the range around the zero point (centralposition of the steering wheel) so as to reduce the energy consumption.Within this range, changes, e.g., changes in the pressure increase, areparticularly noticeable. Consequently, the control behavior will varystrongly between one steering system and another steering system. Inaddition, temperature variations influence the viscosity of the oil inthe steering system. Oil has a high viscosity at low temperatures. Thiscauses an increased circulation pressure in the steering system which,in turn, influences the speed and current of the motor. If one of thesevalues is acquired, the control electronics interpret this as a steeringmovement. This in turn causes the electronics to switch the motor intothe incorrect operating state. This results in an increased energyconsumption and a higher noise level than necessary.

According to one advantageous proposal of the invention, the powerconsumption of the electric motor is monitored as an operatingparameter. According to additional proposals of the invention, thehydraulic pressure and the rotation of the shaft of the electric motorcan be monitored.

It is particularly advantageous that the minimum power consumption bedetermined within regular time intervals and that this value be used asthe basis for calculating a switching point between the powerconsumption ranges of the electric motor.

The evaluation of the current signal serves for determining minimumvalues that indicate to which level the current has dropped thus far.The comparison value is, for example, now briefly set above the minimumvalue. If this comparison value is exceeded, said fact is detected as asteering deflection and the motor is supplied with the correspondingpower. Once the steering system warms up, the viscosity of the oil andconsequently the circulation pressure and the current decrease or thespeed increases. This is detected by means of the minimum value and thecomparison value is set anew. This measure makes it possible to alwaysattain a constant sensitivity of the control unit, namely independent ofvariations in the steering system or temperature.

According to one additional advantageous proposal of the invention, thepower supply of the electric motor is limited once at least one selectedoperating parameter has exceeded a fixed upper reference value for apredetermined duration. It was explained previously that it isconsidered disadvantageous that known power-assisted steering systemsconsume excess power if the steering wheel is turned to either limitstop in order to generate a high pressure because the steering valve isclosed. Moreover, this does not provide any advantages, but rather leadsto an intense heating of the motor and the electronics such that saidcomponents can be destroyed very rapidly if no temperature cut-off isprovided.

It is advantageous that the current be continuously monitored. If thelimiting value is exceeded for longer than three seconds for example,current limiting that limits the current is activated. This measuresignificantly reduces the power losses, but simultaneously causes themotor to be almost blocked. However, this is not critical because thesteering system is already situated at one limit stop. The movement ofthe rotor of the motor is now monitored,. e.g., by means of Hallsensors. Once the motor again turns more rapidly, this means that thesteering system is no longer situated at either limit stop and thecurrent limitation can be switched off again such that the steeringsystem is returned to the normal operating state.

It is particularly advantageous that the calculation of the referencevalues be carried out in a clocked fashion. It is particularlyadvantageous that monitored operating parameters that are functionallyrelated be subjected to a plausibility check. This can, for example, becarried out with the aid of stored value tables.

Inter-related operating parameters include for example, the hydraulicpressure and the rotational speed of the shaft of the electric motor, aswell as the rotational speed of the pump shaft, the temperature and thelike. Such operating parameters are measured and it is determined, e.g.,with the aid of value tables, whether the measured combination of valuesis plausible. If this is not the case, one can conclude that afunctional problem is present and this functional problem can, forexample, be pinpointed.

The invention discloses a power-assisted steering system and a methodwhich make it possible to design a power-assisted steering system in amore comfortable fashion and attain extended service life.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional details and advantages are disclosed in the followingdescription of the enclosed figures that schematically illustrate twoembodiments of a power-assisted steering system according to theinvention. The figures show:

FIG. 1: a hydraulic diagram and a circuit diagram of a power-assistedsteering system with a current sensor;

FIG. 2: a detailed representation of the electronics with a currentsensor which is only illustrated in the form of a block diagram in FIG.1;

FIG. 3: a circuit diagram of a power-assisted steering system as well asthe steering gear with a proximity sensor which are illustrated in theform of a longitudinal cross section;

FIG. 4: a flow chart that illustrates the processing of the sensorsignal in the electronic control unit;

FIG. 5: a diagram that illustrates the time history of the motor voltagein dependence on the current flowing through the switching transistorand the load on the pump;

FIG. 6: a flow chart that illustrates the processing and defining of thereference values for switching into a higher power consumption range;

FIG. 7: a flow chart that illustrates the processing of the referencevalues for switching into a lower power consumption range, and

FIG. 8: a flow chart that illustrates the activation of currentlimiting.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A power-assisted steering system is used in association with a not-shownmechanical steering system in order to reduce the steering torque to beapplied via the steering wheel. This is realized by means of a hydrauliccylinder 1, the piston rod 2 of which is connected to a steering tie rod3 (illustrated in FIG. 3). The chambers 4 and 5 of the hydrauliccylinder 1 are charged with hydraulic fluid in dependence on thesteering deflection by means of a pump 7 via a servo-valve 6, throughthe lines 8 and 9.

The hydraulic diagram in FIG. 1 shows that the hydraulic fluid is pumpedfrom a reservoir 10 to the hydraulic cylinder 1 via the servo-valve 6 bymeans of the pump 7. If the hydraulic fluid is, for example, pumped intothe chamber 4 via the line 8, the piston 11 that is arranged on a pistonrod 2 is moved toward the left such that the hydraulic fluid isdisplaced from the chamber 5 back to the servo-valve 6 via the line 9.This hydraulic fluid then flows back into the reservoir 10. In theneutral position of the hydraulic cylinder 1 and the servo-valve 6 whichis illustrated in FIG. 1, no hydraulic fluid reaches the hydrauliccylinder 1. The hydraulic fluid is only pumped from the reservoir 10back into the reservoir 10 through the servo-valve 6 by means of thepump 7. A pressure control valve 12 that produces a short circuitbetween the pump 7 and the reservoir 10 for instances in which thehydraulic fluid cannot be discharged via the servo-valve 6 or theservo-valve 6 and the hydraulic cylinder 1 is also arranged in thehydraulic diagram for safety reasons.

The pump 7 that serves for conveying the hydraulic fluid is driven by anelectric motor 13 that is supplied with voltage by the battery 14 of themotor vehicle via electronics 21. A detailed illustration of theelectronics 21 is shown in FIG. 2. In order to realize a voltage supplyof the electric motor 13 that depends on the respective requirements, acurrent sensor 15 is provided which measures the power requirement ofthe pump 7 and conveys said power requirement to a switching transistor16 via an electronic control unit 17. The basic function of thiselectronic control unit 17 is shown in the flow chart according to FIG.4.

Instead of the current sensor 15 illustrated in FIG. 2, thepower-assisted steering system according to FIG. 3 can also be equippedwith a proximity sensor 18 that scans the position of a suitablestructural component in the steering gear 19. The information of theproximity sensor 18 is again fed to the electronic control unit 17 whichevaluates this signal in accordance with the flow chart illustrated inFIG. 4 and determines the supply voltage of the electric motor 18therefrom.

According to FIG. 4, the electronic control unit 17 operates as follows:

At the beginning, the motor voltage U_(mot) is low, i.e., the pump 7operates in standby mode. The signal I that is fed to the switchingtransistor 16 by the current sensor 15 via the electronic control unit17 is compared with an adjusted switch-on limiting value Ie, with themotor 13 or the pump 7 being connected to the full motor voltage U_(mot)if the limiting value Ie is exceeded. This motor voltage U_(mot) ismaintained until a signal I that is smaller than a switch-off value Iais fed from the current sensor 15 to the electronic control unit 17. Ifthe signal falls below this switch-off limiting value Ia, the motorvoltage U_(mot) is switched back such that the pump 7 operates instandby mode. A minimum duration t_(min) is provided so as to preventthe steering comfort from being impaired due to the rapid alternationbetween switching on and switching off the full motor voltage U_(mot).The motor voltage U_(mot) is only switched back again once this minimumduration t_(min) has been exceeded.

The dependence of the motor voltage U_(mot) on the current I which isrealized by means of the switching transistor 16 as well as the load onthe pump 7 are illustrated in FIG. 5. These diagrams show that the fullmotor voltage U_(mot) is made available as soon as the current signalexceeds the switch-on limiting value Ie. The motor voltage U_(mot) isswitched back into standby mode if the current signal I falls short ofthe switch-off limiting value Ia for a minimum duration t_(min).

A power-assisted steering system that is designed in this fashioninsures that the power-assist of the steering system is also guaranteedif the motor of the motor vehicle is not in operation because the pump 7for supplying the servo-valve 6 or the hydraulic cylinder 1 withhydraulic fluid is driven by the electric motor 13 which receives itssupply voltage from the battery 14. On the other hand, the utilizationof the current sensor 15 or the proximity sensor 18 in association withthe electronic control unit 17 and the switching transistor 16 make itpossible to realize a power consumption of the pump 7 that depends onthe respective requirement such that an increased service life of thecomponents, a reduced noise development and a reduced energy consumptionare attained. It is particularly advantageous that the electronics 21which comprise the current sensor 15 or a pressure sensor and theelectronic control unit 17 as well as the switching transistor 16 besubsequently installed into existing power-assisted steering systems.

The flow charts illustrated in FIGS. 6, 7 and 8 show one example forcontrolling the method according to the invention. FIG. 6 shows thebasic program and FIGS. 7 and 8 show the flow charts for thesubprograms. Originating from the starting point according to FIG. 6,the motor is initially switched into an operating state of low powerconsumption. The minimum current value MW is measured by means of thesensors and used for calculating the lower and upper threshold Su, So.Subsequently, it is ascertained if the present motor current exceeds thelower threshold. As long as this is not the case, it is determinedwhether the motor current falls below the thus far measured minimummotor current MW. If this is the case, the control returns to the pointat which MW is measured. However, if this is not the case, the routinereturns to the point at which it is determined whether the present motorcurrent exceeds the lower threshold Su. The two aforementioned routinesare continued until it is determined that the present motor currentexceeds the lower threshold Su. In this case, the so-called "motorhigh"-routine illustrated in FIG. 7 is activated. During this routine,the operating mode of the motor is initially set to high in a firststep. Subsequently, it is determined whether the current is below theupper threshold So. A time interval is initially started. Subsequently,it is determined whether the current has again exceeded the upperthreshold So (the steering wheel is turned again). If this is the case,the time interval is reset again and the evaluation is repeated. If thecurrent now again falls below the upper threshold, the time interval isstarted anew and it is determined whether the current has again exceededthe upper threshold So. If this is not the case, it is determinedwhether the time interval is concluded, i.e., "delay elapsed"? As longas this is not the case, the evaluation is repeated. If the currentagain rises above the upper limit within this time interval, the timeinterval is again reset and the upper operating state is maintained.However, if the time interval check has been concluded, the loweroperating state is adjusted and the routine returns to the main routine.

During the routine illustrated in FIG. 7, as long as the present motorcurrent is above the lower threshold Su, it is regularly determinedwhether the motor current is still below the upper threshold So. Theroutine "current limiting" which is illustrated in FIG. 8 is activatedat the time at which the present motor current exceeds this upperthreshold.

In the routine shown in FIG. 8, it is initially determined whether theupper threshold So has been exceeded for three seconds. If this is notthe case, i.e., if the present motor current again falls below thisthreshold within three seconds, the routine returns to the "motor high"routine according to FIG. 7. However, if the present motor currentremains above the upper threshold So for three seconds, a currentlimitation that only supplies the motor with a lower current isactivated. A sensor evaluation, e.g., the evaluation of a Hall sensor,allows the determination of whether the shaft of the electric motorrotates or the motor is blocked. As long as the motor is blocked, saidmotor remains in its state and the current limitation remains active.However, if the shaft of the electric motor rotates, this means that theoperating conditions have changed, i.e., the pressure has dropped. Thecurrent limitation is deactivated again and the routine returns to the"motor high" routine.

List of Reference Symbols

1 Hydraulic cylinder

2 Piston rod

3 Steering tie rod

4 Chamber

5 Chamber

6 Servo-valve

7 Pump

8 Line

9 Line

10 Reservoir

11 Piston

12 Pressure control valve

13 Electric motor

14 Battery

15 Current sensor

16 Switching transistor

17 Control unit

18 Proximity sensor

19 Steering gear

20 Steering gear

21 Electronics

U_(mot) =Motor voltage

I=Current

Ie=Switch-on limiting value

Ia=Switch-off limiting value

t_(min) =Minimum duration

We claim:
 1. A power-assisted steering system for a vehicle comprising:ahydraulic motor for providing vehicle steering assistance; an opencenter servo-valve for supplying said hydraulic motor with hydraulicfluid; a hydraulic pump for supplying hydraulic fluid to saidservo-valve and to said hydraulic motor; an electric motor for drivingsaid hydraulic pump; and means for controlling said hydraulic pump bycontrolling electrical power consumed by said electric motor, said meansincluding at least one sensor which repetitively monitors at least oneoperating parameter of said power-assisted steering system, said meansfurther including at least one microprocessor which repetitivelycalculates upper and lower threshold values that vary as a function ofsaid at least one operating parameter; said means controlling theelectrical power consumed by said electric motor based on a comparisonof reference values calculated as a function of an initial value of saidat least one operating parameter with a subsequent value of said atleast one operating parameter; said at least one microprocessor beingoperable to recalculate said threshold values as a function of saidsubsequent value of said at least one operating parameter based on acomparison of said initial value of said at least one operatingparameter with said subsequent value.
 2. The power-assisted steeringsystem as set forth in claim 1 wherein said at least one sensorcomprises a current sensor which measures the electrical power requiredby said hydraulic pump.
 3. The power-assisted steering system as setforth in claim 2 wherein said at least one microprocessor determines aminimum power consumption via said current sensor in order to calculatesaid threshold values for switching said electric motor between powerconsumption ranges.
 4. The power-assisted steering system as set forthin claim 3 wherein said at least one sensor further comprises a pressuresensor which measures pressure changes in said servo-valve.
 5. Thepower-assisted steering system as set forth in claim 1 wherein said atleast one sensor comprises a proximity sensor which measures therelative position of a structural component of the power-assistedsteering system.
 6. The power-assisted steering system as set forth inclaim 1 wherein said electric motor is supplied with a minimum voltageand operates in a standby mode when either said hydraulic motor or saidservo-valve are in a neutral position.
 7. The power-assisted steeringsystem as set forth in claim 1 wherein said at least one microprocessorevaluates said at least one operating parameter and determines theelectrical power consumption of said electric motor therefrom.
 8. Thepower-assisted steering system as set forth in claim 1 wherein said atleast one microprocessor activates a current limiting means for limitingthe current to said electric motor if said at least one operatingparameter exceeds one of said threshold values for a predeterminedduration.
 9. The power-assisted steering system as set forth in claim 8wherein said at least one sensor includes a Hall-effect sensor fordetermining whether said electric motor is blocked.
 10. Thepower-assisted steering system as set forth in claim 1 wherein said atleast one microprocessor monitors at least two operating parameters ofsaid power-assisted steering system being monitored by at least twosensors as part of a plausibility check.
 11. A method for controlling anelectrical motor which drives a hydraulic pump in a power-assistedsteering system, said method comprising the steps of:(a) providing meansfor controlling electrical power consumed by the electric motor, saidmeans including at least one sensor for monitoring at least oneoperating parameter of the power-assisted steering system and at leastone microprocessor for calculating upper and lower threshold valueswhich vary as a function of said at least one operating parameter; (b)measuring an initial value of said at least one operating parameter; (c)calculating said threshold values as a function of said initial value ofsaid at least one operating parameter; (d) measuring a subsequent valueof said at least one operating parameter; (e) comparing said subsequentvalue with said threshold values; (f) adjusting the electrical power tothe electric motor based on the results of step (e); (g) comparing saidsubsequent value of said at least one operating parameter with saidinitial value; (h) recalculating said threshold values if said initialvalue exceeds said subsequent value, and then returning to step (d); and(i) returning to step (e) without recalculating said threshold values ifsaid subsequent value exceeds said initial value.
 12. The method ofclaim 11 wherein said at least one operating parameter being monitoredis electrical power consumed by the electric motor.
 13. The method ofclaim 12 wherein step (f) includes limiting the electrical powersupplied to the electric motor if said subsequent value exceeds one ofsaid threshold values for a predetermined duration.